isomerizations under mild conditions [67]. It was hypothesized that increasing the
steric bulk at the pyridine site of the pincer catalyst might significantly improve the
selectivity of the reaction.
The synthetic strategy leading to these ligands is relatively modular because it
relies on robust Suzuki coupling, followed by a series of trivial functional group
modifications. For example, the cross-coupling between 2-bromo-6-(tert-butyl)pyridine (15) and 3-(hydroxymethyl)phenylboronic acids (16), followed by a two-step
modification of the hydroxymethyl sidearm in the corresponding 2-(tert-butyl)-6{3-[(di-tert-butylphosphino)methyl]phenyl}pyridine (17), leads to the desired 18 in
good overall yield. The oxidative carbometalation of the latter with [IrCl(COD)] 2
leads to the formation of PCN iridium hydride 19 [68]. Its successive reaction with
NaOtBu in toluene leads to the creation of a new complex 20, where the iridium
center lacks the coordination of other ligands except for the agostic interaction with
the appended tert-butyl group. Although X-ray crystallographic analysis of this
agostic complex was not provided, the spectroscopic data are consistent with the
suggested structure (Scheme 5).
The new agostic complex 20 was evaluated for the isomerization of linear
terminal alkenes. It was expected that the agostic bonds would dissociate readily
to generate the catalytically active 14-electron species. For example, the catalytic
isomerization of 1-octene using 20 as a catalyst occurred at RT, resulting in the
formation of 94% (E)-2-octene (E/Z ¼ 51:1) in 1 h. High conversions and high
regio- and stereoselectivities were obtained for a broad spectrum of substrates. For
example, industrially valuable linear C 6 -, C 7 -, C 10 -, and C 16 -alkenes were converted
into (E)-2-alkenes with high selectivities. Importantly, this catalyst system was
tolerant of various functional groups. Thus, terminal alkenes bearing free and
Scheme 4 The NCP-type Ir pincer catalysts for alkene isomerization
Scheme 5 Synthesis of the complex 20 possessing agostic functionality
Cooperative Reactivity by Pincer-Type Complexes Possessing Secondary. . .
101
steric bulk at the pyridine site of the pincer catalyst might significantly improve the
selectivity of the reaction.
The synthetic strategy leading to these ligands is relatively modular because it
relies on robust Suzuki coupling, followed by a series of trivial functional group
modifications. For example, the cross-coupling between 2-bromo-6-(tert-butyl)pyridine (15) and 3-(hydroxymethyl)phenylboronic acids (16), followed by a two-step
modification of the hydroxymethyl sidearm in the corresponding 2-(tert-butyl)-6{3-[(di-tert-butylphosphino)methyl]phenyl}pyridine (17), leads to the desired 18 in
good overall yield. The oxidative carbometalation of the latter with [IrCl(COD)] 2
leads to the formation of PCN iridium hydride 19 [68]. Its successive reaction with
NaOtBu in toluene leads to the creation of a new complex 20, where the iridium
center lacks the coordination of other ligands except for the agostic interaction with
the appended tert-butyl group. Although X-ray crystallographic analysis of this
agostic complex was not provided, the spectroscopic data are consistent with the
suggested structure (Scheme 5).
The new agostic complex 20 was evaluated for the isomerization of linear
terminal alkenes. It was expected that the agostic bonds would dissociate readily
to generate the catalytically active 14-electron species. For example, the catalytic
isomerization of 1-octene using 20 as a catalyst occurred at RT, resulting in the
formation of 94% (E)-2-octene (E/Z ¼ 51:1) in 1 h. High conversions and high
regio- and stereoselectivities were obtained for a broad spectrum of substrates. For
example, industrially valuable linear C 6 -, C 7 -, C 10 -, and C 16 -alkenes were converted
into (E)-2-alkenes with high selectivities. Importantly, this catalyst system was
tolerant of various functional groups. Thus, terminal alkenes bearing free and
Scheme 4 The NCP-type Ir pincer catalysts for alkene isomerization
Scheme 5 Synthesis of the complex 20 possessing agostic functionality
Cooperative Reactivity by Pincer-Type Complexes Possessing Secondary. . .
101